1. Field of the Invention
This invention relates to a sonic positioning system, in particular, a sonic positioning system for precisely determining the position of an automatically guided vehicle relative to the vehicle's environment.
2. Description of the Related Art
A significant problem hindering the development of automatically guided vehicles (AGV(s)) is that of determining position of a given AGV within the AGV's normal operating space. It is to be understood that the present invention is not limited to the position determination of AGVs but determines the position of any object utilizing a system constructed in accordance with this invention. For example, a robotic AGV whose function is to store and retrieve objects in a large warehouse must navigate the aisles and corridors of the warehouse and place objects into and retrieve objects from appropriate bins, or an automated vacuum cleaner must traverse an entire floor space without undue redundancy.
One method that is old in the art for determining the position of an AGV within a given work space is one in which the AGV tracks its own movement by maintaining records of position, direction and movement information. Such a method is commonly called "dead reckoning." In dead reckoning, the new position is determined from the AGV's previous position, the AGV's direction and the distance travelled by the AGV since its last position was determined. An AGV's new direction is calculated from the AGV's previous direction and the change in direction caused by the turning of the AGV. Each new calculation of position and/or direction introduces a small error. For the first few steps, dead reckoning is fairly accurate, however, as an AGV constantly recalculates the position and direction, errors quickly accumulate to significant levels and position and direction information of the AGV quickly becomes meaningless.
Another way that prior art AGVs determine position is by following lines from one known place to another known place. In a line-following system, every path the AGV is capable of taking is marked by painting lines on the floor which the AGV optically senses and follows, or by laying guide wires under the floor which the AGV electronically or otherwise senses and follows. Such a line-following system requires extensive preparation and is difficult to change or adapt. Additionally, causing an AGV to lose track of the line followed by the AGV disrupts the AGV to such an extent that the AGV cannot resume transit to the AGV's destination.
A third method by which AGVs attempt to navigate within their environment is through the use of video cameras or sonic transducers with which the AGV attempts to "see" the surrounding environment. The AGV determines position within the workspace by attempting to recognize landmarks within the workspace. However, such a system is of limited utility in environments which change with time. Current landmark recognition systems look for large, high-contrast objects to serve as landmarks. For example, for an optical landmark recognition system for guiding a vehicle through the streets of a city, other vehicles and pedestrians may have more distinctive shapes and color contrasts than traffic signals, traffic signs and signs bearing the names of streets. Selecting other vehicles and pedestrians as landmarks would render such a system useless since other vehicles and pedestrians are mobile and will have moved before the AGV passes by the position identified by such landmarks at some later time. Furthermore, landmark recognition systems often require excessively complex artificial intelligence logic. The time required to process a video image and to scan that image for any of a number of landmark images is substantial, causing the AGV to travel slowly and to hesitate frequently in repeated attempts to recognize landmarks. Additionally, the complexity of such computations requires a significant amount of computer hardware, making such a system potentially viable only for larger, heavier and more costly AGVs.
Similar to the optical landmark recognition system is the sonic echo location system in which a sonic chirp is generated and the time it takes the chirp to echo back is measured to calculate the distance to the object from which the chirp is reflected. Such a system is relatively accurate in judging the distance to an object as the speed of sound through air is essentially independent of relative humidity and air pressure and only slightly affected by temperature. While such a system may be quite useful for the purpose of collision avoidance as objects in an AGV's path may be effectively detected by an echo-location system, such a system provides no information as to the AGV's current position or direction. Additionally, inaccurate or misleading information may result from multiple chirp echoes being detected by such a system in complex surroundings (especially in corners) or from sonic waves reflecting off of smooth surfaces and not returning to the sonic source (e.g., a smooth surface such as glass that is as little as 7.degree. from perpendicular from the direction of sound waves from the AGV will reflect the waves away from the AGV so as to go undetected, giving an inaccurate "view" of the AGV's surroundings).
Another way AGVs may currently determine position is by signalling member satellites of the global positioning system (GPS), now being developed and implemented by the United States Department of Defense, and calculating from the responses of those satellites the latitude and longitude of the AGV's current position. However, such a system is generally too inaccurate for most AGV applications. Results are currently accurate to within 3 meters at best and usually only to within 15 to 30 meters. Such large errors are unacceptable as errors of such magnitude may cause an AGV to enter the wrong room or retrieve objects from the wrong bin.
In addition to inadequate accuracy, GPS systems are generally too slow for most AGV applications. It generally takes as long as 3 minutes to determine a position. Preferably, AGVs should move about relatively quickly and without hesitation. While some hesitation for determining position is acceptable under certain circumstances, pauses of up to 3 minutes are clearly unreasonable for many AGV applications. Some GPS systems offer position updates every two and a half seconds. Even such systems are too slow for a continuously and quickly moving AGV operating within a small area.
Additionally, GPS systems are generally less useful indoors as the radio frequency field is often distorted by the geometry and contents of buildings.
What is needed is a system for determining the position of an AGV that is accurate to within a few centimeters in a given operating space. Such a system should produce position information quickly, e.g., on the order of 200 milliseconds or less. Furthermore, such a system should be inexpensive and compact so that it may be used in small household robots.